1. Field of the Invention
The present invention relates to an image forming apparatus of an electro photographic system or an electrographic recording system and a control method thereof.
2. Description of the Related Art
An image forming apparatus such as a copy machine and a printer is common, which forms an electrostatic latent image on a photosensitive drum that is an electrostatic image bearing member by using a laser beam. Such an image forming apparatus modulates the laser beam oscillated from a laser oscillator depending on an image signal and irradiates the laser beam to a polygonal mirror that is rapidly rotating. This laser beam is reflected by the polygonal mirror, and in accordance with the rotation of the mirror, the laser beam is deflected and scanned along an axial direction of the photosensitive drum. Further, this laser beam is focused on the photosensitive drum via an imaging lens to form an electrostatic latent image according to each scanning line of the image.
This kind of image forming apparatus has been required to realize the image formation with a high resolution from a view of reproducibility of minute characters and uniformity such as proportion. In JP-A-10-288874, a method for forming a stable quality of an image is disclosed. In other words, it is necessary to control an applied bias (Vg) necessary for obtaining a predetermined electric potential at a dark section (Vd) and an exposure intensity (LP) of an optical beam necessary for obtaining a predetermined electric potential of a bright section (VL).
Nevertheless, in a conventional image forming apparatus, adjustment of the applied bias (Vg) and adjustment of the exposure intensity of the optical beam (LP) have been carried out by different methods. For example, in order to adjust the applied bias (Vg), Vg is determined by rotating the photosensitive drum three rounds (three revolutions) with different applied biases respectively, and then, in order to determine the exposure intensity of the optical beam, further, the exposure intensity (LP) is determined by rotating the photosensitive drum four rounds with different exposure intensity respectively. It is necessary to rotate the photosensitive drum for the number of revolutions corresponding to the different applied biases and the exposure intensities of the different optical beams. In other words, in this case, for adjustment, the photosensitive drum should be rotated seven times in total. As a matter of course, since the image formation cannot be carried out during execution of a sequence for this adjustment, a user would be waiting.
Further, in order to obtain a high quality picture and a highly stable image forming apparatus, it is necessary to increase a frequency of the sequence of this adjustment. However, if the frequency of the adjustment sequence is increased, it is feared that a standby time of the user is further increased.